1. Field of the Invention
This invention relates to an improved method and apparatus for coherent phase demodulation for use at communication terminals receiving a binary phase modulated carrier. More particularly, an improved coherent phase demodulator generates stored fundamental and double frequency phase angle vector signals in response to phase coded signals derived from sampling subdata symbol segments of the carrier. The fundamental vector signals representing each carrier segment are compared with reference fundamental phase angle vector signals derived from the double frequency vector signals representing many segments to produce correlation signals which are summed for each data symbol.
2. State of the Prior Art
In carrier communication systems generally, binary coded data signals modulate a carrier when it is transmitted and then upon the signal being received, it must be detected so that the data bit information is quickly, reliably and accurately recovered after the carrier signal has been subjected to and combined with attenuation, signal noise and other interference and distortion effects causing serious degradation of the signal. In power line communication systems of the automated distribution type, the transmitted carrier signal is particularly subjected to severely interfering impulse and broad-band noise conditions due to being transmitted with electric power conducted along power line distribution conductors. Another consideration in power line communication systems is that the system often includes one or more transmitting stations, optional signal repeaters and large numbers of remote terminals each being located at a power customer location. Message encoded carrier signals are often sent in alternative addressing modes to be received concurrently at large groups of remote terminals or to be received at individual repeaters or terminals. When multiple frequency carrier signals are utilized, substantial complexity can be involved in frequency isolation and discrimination and in avoidance of mutual signal interference. In using single frequency tones or carrier signals, it is important that a receiving terminal or repeater quickly detect the carrier signal since many messages may be time multiplexed for selectively addressing different ones of the terminals or repeaters. It has been found that coherent detection and demodulation systems utilizing a pilot or unmodulated synchronizing signal or combined encoded data and synchronization pulses further adds to the complexity of power line communications equipment. While it is known to use the power signals, typically occurring at fifty or sixty Hz, for carrier synchronization, observed discontinuities in the electric power line paths, such as due to phase transitions between polyphase lines and the like, do not always allow the power signals to be a reliable external synchronization means although an available one.
In non-coherent detection systems, such as those often used for frequency shift keyed carrier signals, the amplitude envelope of the carrier signal is frequently used to establish an indication of a start of message. Noise impulses and spikes can often be the source of erroneous start of message indications and cause the affected receivers to attempt address decode operations and other ambiguous operations causing unsatisfactory performance. Accordingly, the general demodulator requirements for receiving terminals of power line carrier communication systems include optimum detection of the carrier signals in the presence of noise and other interfering signals, selection of the basis for detecting the presence or absence of the carrier signal, little or slight deviation of the receiver and detector operation from an ideal performance, synchronization of the receiver and detector with the carrier signal, synchronization with each data bit of the encoded message information, and polarity synchronization with the binary states of the data bits so as to provide a final error free recognition and reconstitution of the message information data bits as originally transmitted. To optimally satsify the foregoing requirements, phase modulated carrier signals, being of the phase shift keyed (PSK) or coherent phase shift keyed (CPSK) type, are utilized to recover the encoded carrier data contained in the carrier phase relationships so that no carrier envelope amplitudes have to be observed. Signal conditioning of received phase modulated carrier signals includes limiting and clipping of noise spikes in a wide bandwidth followed by amplification and signal limiting in a narrow bandwidth so that the reconstituted carrier at a phase demodulator input is less susceptible to broadband and impulse noise in many applications. Often, in the detection of coherent phase modulation signals, a locally generated signal or local oscillator is required to be synchronized with the incoming signal before correlation can be started. Substantial time can be required prior to signal processing for detection and demodulation of the received data.
Another general technique of phase detection is a so-called differential phase shift keyed type in which the incoming signal is applied to a delay line and then input to a phase detector along with the original signal for comparison with the immediately preceding data bit. Each data bit is processed to compare with the preceding data bit so that it will output a polarity which is the same or which is opposite of the preceding data bit. One disadvantage of this type of phase detection is that the reference uses information included in only a single bit and includes as much noise as is included in the data bit being processed, resulting in a degradation in performance. The differential phase shift keyed systems are sometimes operated at low data rates to assure more accurate and error free operation which in turn causes a limitation in the quantity of data than can be transmitted during a predetermined time interval.
With aforementioned considerations and requirements of phase demodulators in mind, the present invention is directed to a coherent phase demodulator having increased simple and reliable operation with optimum performance utilizing a minimum of operations which are uniquely combined and controlled to produce a system particularly advantageous for use at receiving communication terminals of a power line communication system as summarized hereinafter.